Identification of Dynamic Active Sites Among Cu Species Derived from MOFs@CuPc for Electrocatalytic Nitrate Reduction Reaction to Ammonia

Highlights Cu species with tunable loading supported on N-doped TiO2/C were successfully fabricated utilizing MOFs@CuPc precursors via the pre-anchor and post-pyrolysis strategy. Cu species with tunable loading supported on N-doped TiO2/C were successfully fabricated utilizing MOFs@CuPc precursors via the pre-anchor and post-pyrolysis strategy. Restructured CuN4&Cu4 performed the highest NH3 yield (88.2 mmol h−1 gcata−1) and FE (~94.3%) at − 0.75 V due to optimal adsorption of NO3− and rapid conversion of the key intermediates. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-023-01091-9.


S1.2 General Characterizations
Powder X-ray diffraction (PXRD) patterns of the obtained samples were acquired on a Shimadzu XRD-6000 diffractometer with Cu Kα radiation (λ = 1.54056 Å), the scan rate was set as 5 ° min -1 under a step of 0.02°. Rietveld refinements were performed using the GSAS program with the EXPGUI interface [S1, S2], and the PXRD data were collected with a 1 ° min -1 rate from 5° to 120°. Fourier transform infrared (FT-IR) measurements were performed on a Thermo IS5 FT-IR spectrometer with KBr pellets. Thermogravimetric analysis-differential thermal analysis (TGA-DTA) measurements were performed on a HITACHI STA300 from room temperature to 400 °C with a heating rate of 5 °C min −1 in air. Raman spectra were recorded on a micro-Raman spectrometer consisting of a sample chamber coupled with an RH controller, a gaseous precursor generator, an optical microscope (DMLM; Leica) for observing droplet morphology, and a confocal Raman spectrometer (inVia; Renishaw) with a 514.5 nm argon-ion laser (model LS-514; Laser Physics) as the excitation source with a power of 30 mW. The morphologies of samples were determined with scanning electron microscopy (SEM) and EDS line scan (ZEISS-EVO18, 10 kV), transmission electron microscopy (TEM) and EDX (Talos F200X, ThermoFisher, 300 kV), and aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) at an accelerating voltage of 200 kV (JEM-ARM300F, JEOL). Inductively coupled plasma optical emission spectrometer (ICP-OES) was performed using the Agilent 5800 and dissolved with aqua regia in advance. N2 adsorption-desorption isotherms were degassed at 150 °C for 12 h and subsequently measured at 77 K on a nitrogen adsorption-desorption instrument (Kubo-X1000, China). UV-Vis DRS were recorded by a UV-2600 (Shimadzu) spectrophotometer with quantified as-prepared MOFs-based precursors using BaSO4 as a reference in the wavelength from 200 to 800 nm. Xray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Fisher Scientific ESCALAB 250Xi system with Al Kα radiation (photoelectron energy: 1486.6 eV), and the C1s peak at 284.5 eV was used to calibrate the peak positions. 1 H nuclear magnetic resonance (NMR) spectra were monitored on an Avance III HD 700 MHz spectrometer (Bruker Avance, German).

S1.3 XAS Measurements
X-ray absorption spectra (XAS) were recorded in fluorescence mode at the beamline 4B9A station of the Beijing Synchrotron Radiation Facility (BSRF). The measured data were processed with ATHENA, HAMA and ARTMIS IFEFFIT software packages to obtain X-ray absorption near-edge structure (XANES), Fourier transform extended X-ray absorption fine structure (FT-EXAFS) and wavelet transform extended X-ray absorption fine structure (FT-EXAFS) spectra. Cu and Ti K-edge k 3 -weighted EXAFS spectra were obtained by normalizing with the edge-jump step, and k 3 -weighted χ(k) data at the K edge were Fourier transformed to R space using hanning windows (dk = 1.0 Å −1 ) to separate the EXAFS contributions from different coordination shells and obtain the coordination informations based on Eq. (S1) [S3]. During the calculation of coordination number for Cu cluster or nanoparticle, the Cu 0 proportion obtained by the Linear Combination Fitting (LCF) of XANES spectra was calculated according to Eq. (S2) [S4]. The XANES spectra of Cu foil, Cu2O, and Cu OCP were selected as the standard spectra of Cu 0 , Cu + , and Cu 2+ .

S1.4 Online Differential Electrochemical Mass Spectrometry
Online differential electrochemical mass spectrometry (DEMS) was measured on QAS 100 (Linglu Instruments Co., Ltd.) referring to the electrocatalytic NITRR conditions. The potentiostatic test was performed at -0.75 V vs. RHE, and five cycles were performed when the baseline remained stable to minimize the error of the experiment.

S1.5 Calculation of Electrochemical Active Surface Area (ECSA)
The electrochemical active surface area is determined by CV curves based on the electrochemical double-layer capacitance (Cdl) in a potential window nearly non-Faradaic process with different scan rates of 10, 20, 40, 60, 80, and 100 mV s -1 . Cdl was calculated according the slope of the scan rates vs. geometric current density, and ECSA was obtained with the following Eq. (S3): C s is the specific capacitance for a flat surface at the range of 20 to 60 μF cm -2 , which is frequently assumed to be 40 μF cm -2 .

S1.6 Determination of Ammonia
Indiphenol blue method was used to measure the generated NH3 [S5]. The concentrationabsorbance curves were calibrated using a series of concentrations of NH4Cl solution, and the well-linear relation fitting curve (y = 0.5613x -0.0013, R 2 = 0.9992) was obtained by three times independent calibrations. Typically, electrolyte was taken from the cathodic chamber and diluted to 2 mL, and then 2 mL of 1 M NaOH solution containing 5 wt% C7H6O3 and 5 wt% C6H5Na3O7, 1 mL of 0.05 M NaClO and 0.2 mL of 1 wt% C5FeN6Na2O were added to the above solution. After standing at room temperature for 2 h, the UV-Vis absorption spectra were measured at a wavelength of 655 nm.

S1.7 Determination of Nitrite
The NO2in liquid products were detected with the Griess method [S6]. The following ingredients were added to ultrapure water (25 mL) and stirred to form a clear solution: C6H8N2O2S (1.0 g), C12H16Cl2N2 (0.1 g), and H3PO4 (5 mL), which was employed as the color reagent. The concentration-absorbance curve was calibrated by using a series of standard NaNO2 solutions (y = 3.1967x + 0.0048, R 2 = 0.9999). In detail, a certain quantity of electrolyte obtained from the electrolytic cell was diluted to a 5 mL solution, and then the color reagent (0.1 mL) was subsequently added to the above-mentioned solution and mixed uniformly. After allowing the solution to settle for 10 min, the absorption intensity (540 nm) was measured.

S1.8 Determination of Nitrate
NO3was determined by mixing 0.1 mL of 1 M HCl and 0.01 mL of 0.8 wt% sulfamic acid solution with 5 mL electrolyte diluent, and then the aforementioned solution was measured the absorption intensity at a wavelength of 220 nm (A220) and 275 nm (A275) [S7]. The final absorbance value (A) was evaluated using this equation: A=A 220 -2×A 275 , and different concentrations of NaNO3 solution to calculate the well-linear relation fitting curve (y = 0.2031x + 0.0045, R 2 = 0.9997).

S1.9 Na 15 NO3 Isotope-labeling Experiment
Na 15 NO3 was selected as the feeding N-source to verify the source of produced ammonia. In the experiment of NITRR, 30 mL electrolyte containing 0.5 M Na2SO4 and 50 ppm Na 15 NO3-15 N were added into the cathode cell, and the final NH3 electrolyte were assessed with 700 MHz 1 H NMR.

S1.10 NMR Determination of Ammonia
The produced 14 NH3 or 15 NH3 was also detected with 700 MHz 1 H NMR [S8]. The concentration-integral area (NH4Cl/C4H4O4) curves were calibrated with a series of NH4Cl solution concentrations using 0.04 wt% maleic acid (C4H4O4) in DMSO-d6 as the internal Nano-Micro Letters S4/S29 standard, and the well-linear relation fitting curve was obtained. Electrolyte (0.5 mL) was taken from the cathodic chamber, the pH of the electrolyte was adjusted to ~2 using 0.5 M H2SO4 with a certain volume, and then 0.1 mL 0.04 wt% C4H4O4 in DMSO-d6 was added to the above solution.

S1.11 Calculation of NITRR Performance
Faradaic efficiency of ammonia: Yield rate of ammonia: Faradaic efficiency of nitrite: Yield rate of nitrite: Conversion rate: The selectivity of ammonia: Where is the Faradaic constant (96485 C mol -1 ), is the mass concentration, is the volume of electrolyte (30 mL), is reaction time (1 h), is the total charge traveling through the electrode ( = ∫ d 0 , representes the geometric current density), is the geometric area of carbon paper under the electrolyte (1 cm 2 ).

S1.12 Density Functional Theory Computational Details
All calculations were implemented using the Vienna Abinitio Simulation Package (VASP) code based on Density Functional Theory (DFT) [S9, S10]. For the following calculations of properties, General gradient approximation (GGA) was used with the Perdew-Burke-Ernzerhof (PBE) functional to describe the exchange-correlation potential [S11]. All structural models were entirely relaxed until the ionic Hellmann-Feynman forces were smaller than 0.001 eV/Å, the energy tolerances were less than 10 −6 eV/atom. The interaction between core electrons and valence electrons was described using the frozen-core projector-augmented wave (PAW) method. Wave functions were expanded in a plane wave basis with high energy using plane-wave cutoff energy of 500 eV, and the corresponding gamma-centered Monkhorst-Pack electronic wavevector k-point samplings were denser than 0.2 Å -1 [S12]. Meanwhile, the implicit solvation calculation was performed using VASPsol [S13, S14], a software package that incorporates solvation into VASP within a self-consistent continuum model. Note that the aqueous solution was adopted by use the dielectric constant εb = 80.0, width of dielectric cavity σ = 0.6, the cutoff charge density ρcut = 0.0025 Å −3 and a surface tension parameter of 0.525 meV/Å 2 . The default parameters for VASPsol are used unless otherwise indicated in the text. To verify the reliability of our calculation results, the local density approximation (LDA) function was adopted for describing the exchange-correlation potential [S15]. For Gibbs free energy (ΔG) of the nitride reduction is defined as below: Where ΔEDFT is the DFT electronic energy difference of each step, and T is the temperature (T = 300 K). ΔEZPE and ΔS are the difference in zero-point energy and entropy change, respectively. common path: *NO 2 +2H + +2e -→*NO+H 2 O (S14) * ONH path: *NO+H + +e -→*ONH (S15) *NHO path: *NO+H + +e -→*NHO (S16) *NHO+2H + +e -→*NH 2 OH (S17) *NH 2 OH+H + +e -→*NH 2 +H 2 O (S18) *NH 2 +H + +e -→*NH 3 (S19)

S2 Supplementary Scheme
Scheme S1 Illustration showing the growth process of Cux/NTC Fig. S1 The photograph of electrocatalytic NITRR process The high-resolution XPS spectra of C 1s for aMIL and aMIL@CuPc-2 samples (Fig. S2b) exhibit peaks at 288.8, 286.4, 285.5, and 284.7 eV, which could be assigned to C=C, C-N, C-C, and C=O bonds, while the peaks located at 288.0 eV of aMIL@CuPc-2 and CuPc samples belong to N-C=N [S16, S17]. The high-resolution XPS spectra of O 1s (Fig. S2c) centered at approximately 532.6, 531.5, and 530.4 eV correspond to -OH group, C=O, and Ti-O cluster [16]. The two broad peaks located at about 464 and 459 eV in the high-resolution XPS spectra of Ti 2p (Fig. S2d) correspond to Ti 4+ 2p1/2 and Ti 4+ 2p3/2 [S16].  Fig. S3a shows that there is almost no weight loss for CuPc and aMIL@CuPc-2 samples from room temperature to ~320 º C, which manifests CuPc molecule (β type) possesses well stability after one-pot synthesis. Cu K-edge X-ray absorption near-edge structure (XANES), k 3 -weight Fourier-transformed extended X-ray absorption fine-structure (FT-EXAFS) spectra, and wavelet transform extended X-ray absorption fine-structure (WT-EXAFS) spectra of CuPc and aMIL@CuPc-2 samples (Fig. S3bd) indicate the comparability of Cu atoms about valence states and bonding environments between CuPc and aMIL@CuPc-2 samples. The PXRD patterns of derivatives shows two characteristic peaks at 25.3º and 27.4º , which could be assigned to the mixed phases of Anatase TiO2 (JCPDS: 21-1272) and Rutile TiO2 (JCPDS: 21-1276), respectively. To precisely illuminate the phase component of Anatase and Rutile TiO2, the PXRD pattern of NTC sample was recorded with slower exposing times and wider angles (1 º min -1 , 5º to 120º ), which were carried out with GSAS software to obtain the results of Rietveld refinements (Table S2). Fig. S7 a, b Nitrogen adsorption/desorption isotherms and Raman spectra of NTC, Cu0.7/NTC, Cu1.5/NTC, and Cu3.2/NTC All isotherms (Fig. S7a) belong to reversible type I, with three stages ranging from low relative pressure (P/P0), relative pressure ranging from 0.2 to 0.9, to near saturation pressure [S18]. Brunauer-Emmett-Teller (BET) specific surface area and pore volume (Table S3) showed that Cu1.5/NTC samples had the largest specific surface area and pore volume, which could provide more active sites. Raman spectra of NTC, Cu0.7/NTC, Cu1.5/NTC and Cu3.2/NTC samples (Fig. S7b) have D and G peaks of C at ~1352 and 1591 cm -1 , and the intensity ratio of D to G peak (ID/IG) gradually increases from 0.86 to 0.92. This indicated that the introduction of CuPc in the precursor increased the degree of graphitization of C in the derivatives and enhanced the conductivity of C materials [S19].   (Fig. S9a) show that the Ti valence state of NTC and Cu1.5/NTC samples is +4. The peak in the first shell of P25 sample at 1.58 Å belongs to the Ti-O scattering path, while the peaks of NTC and Cu1.5/NTC samples at 1.58 and 1.57 Å correspond to Ti-O/N scattering path. WT-EXAFS spectra (FIG.  3.13c) show that the maximum WT strength of NTC (5.6 Å -1 ) is smaller than that of P25 (6.3 Å -1 ), indicating that there is coordination between light atoms and Ti for NTC sample (Ti-O/N). However, the maximum WT strength of Cu1.5/NTC is larger than that of NTC and P25, which could result from the strong synergistic effect of Ti-O/N and Cu species [S21].    (Fig. S19ad). The corresponding curve of current density versus different CV sweep speeds and the slope (Fig. S19e) are fitted to obtain the double-layer capacitance (Cdl). According to Eq. (S3), the electrochemical activity specific surface area (AECSA) of NTC, Cu0.7/NTC, Cu1.5/NTC and Cu3.2/NTC are 167.5, 287.5, 362.5 and 215.0 cm 2 ECSA (Fig. S19f). To assess the stability of the NITRR catalyst with CuN4&Cu4, long-term experiment was operated with 6 cycles, and catalyst dispersed on carbon paper would be transferred to the mixed aqueous solution with Ar-saturated 0.5 M Na2SO4 and 50 ppm NaNO3 after quickly moistening with ultrapure water. The XANES LCF spectra and the relative content of variable Cu with different valence state (Fig. S25) show that the content of Cu 2+ decreases from 100% to 79.4% with the negative shift of reaction potential from OCP to -0.95 V, the content of Cu + increases from 0 to 7.8%, while the content of Cu 0 increases from 0 to 12.8%, which indicating that Cu0.7/NTC predominantly occur the transformation from Cu-N4 to Cu + -Nx (x ≤ 3) at-0.75 V and reconstruction clustering behavior of Cu0.7/NTC begins at -0.85 V. The initial Cu 2+ -N4 configuration in Cu0.7/NTC coexists with Cu + -Nx and Cu 0 clusters along with the negative shift of reaction potential, but the Cu 2+ -N4 configuration is the main form of existence. Fig. S26 WT-EXAFS at Cu K-edge for Cu0.7/NTC with different potential Fig. S27 a, b The fitting spectra of R and k spaces for Cu3.2/NTC with different potential